I am using Thunderbird on Kubuntu. On default, Thunderbird will use the platform's file picker dialog, which is broken on KDE. Once I click a single file, the mouse curor is not released and if I move it around, a square is drawn on the screen, which is typically used to select multiple files. Clicking a second time or pressing escape will cause even more problems.

In the past, this issue could be fixed by setting the option ui.allow_platform_file_picker to false in Thunderbird's advanced options. This caused the mail client to use a built-in dialog, which is very simple but works as expected.

However, with the latest Thunderbird update this option is no longer respected. It is still set to false, but now Thunderbird uses the platform's dialog anyway.